The invention relates to a yarn quality assurance method when manufacturing a multifilament yarn and a yarn processing machine for implementing the yarn quality assurance method. The yarn quality assurance method comprises the process steps of taking off at least one thread from respectively one feeding spool and supplying the yarn via a run of thread into a multifilament yarn manufacturing unit, wherein the yarns are held at a thread tension. The yarn quality assurance method is suitable in principle for use in any multifilament yarn manufacturing method e.g., in draw-winding or weaving, but especially for use in texturing methods, especially false twist texturing methods and/or air covering methods.
Machines with an air-covering nozzle (air-covering machine) are used to process filament yarns together permanently to form a multi-component yarn (air-covering yarn) by means of an air jet (interlacing jet). In this case, at least one covering thread (fancy yarn) is joined to a core thread as components. The aim of this process is to achieve joining knots in the multi-component yarn which are as uniform as possible and thus join the components together whereby the multifilament yarn produced has good mechanical and structural elasticity. An example of an air-covering yarn is disclosed in U.S. Pat. No. 6,405,519 B1. The core thread generally comprises an elastomer-containing highly elastic yarn, e.g. Lycra®. The covering threads can comprise various fancy yarns. The yarns, that is the covering threads and the core thread are supplied via feeder rolls e.g. godet rolls, to an air-covering nozzle. After the multi-component yarn has passed through the air-covering nozzle, the multi-component yarn is taken up by a spool onto which said multi-component yarn is wound.
In this case, the elastic yarn is unwound from a feeding spool positively and tangentially, with the aid of special feeder rolls (tangential take-off). In order to execute this operation, at least one additional drive is required. In order to tangentially unwind the feeding spool of the elastic yarn, said spool is positioned with its axis parallel to the axes of the feeder rolls used for the unwinding. When the feeding spool is empty, the complete manufacturing process must be stopped so that the feeding spool can be replaced. In order to interrupt the production process only as briefly as possible when the feeding spool needs to be changed, e.g., when the current feeding spool is used up, complex systems have been developed for changing the feeding spool automatically. Such a system is disclosed for example in WO 2004/035446. For tangential take-off a distinction is made between two variants. In free-running tangential take-off the feeding spool sits on a freely rotating tube without is own drive. Freely-rotating take-off can only be used at low process speeds. In driven tangential take-off the rotation of the feeding spool is driven via supporting rollers. This driven tangential take-off can be used for high production speeds with elastic yarns.
The covering threads can be produced, for example using a known yarn processing machine for processing filament yarns with an air-texturing nozzle (air-texturing machine). The manufacture of the covering threads and the processing. e.g. using an air-texturing nozzle, is usually carried out by a single machine. Such an air-texturing machine is disclosed in DE 39 09 516 A1. Air-texturing machines are used to permanently crimp smooth structureless filament yarns. In this case, a plurality of feed yarns (core threads) can be processed with fancy yarns of different tradition to form a textured yarn. In this process the filament yarns are uniformly crimped and if necessary arranged around the feed yarns. The covering threads can comprise various fancy threads. The filament yarns, that is the covering threads and the core threads, are supplied to an air-texturing nozzle via feeder rolls, e.g. godet rolls. After the yarns have passed through the air-texturing nozzle, the product fancy yarn consisting of a plurality of components (multi-component yarn) is taken off from a spool onto which the multi-component yarn is wound. Before the spooling the multi-component yarn can be stretched again, fixed, shrunk and/or finished.
It is further known to manufacture the covering threads using a yarn processing machine for implementing the false twist texturing method. This method is known as torsion crimping. In this case, the filament yarn is given a so-called false twist by a false twist spindle between two pairs of cylinders, namely feeder rolls and take-off rollers, and this false twist is fixed in its capillary threads by heating the filament yarn using its thermoplastic properties. After cooling the latent torsional forces have an effect and result in crimping of the product fancy yarn.
For this purpose, the multifilament yarn (thread) is generally unwound from a spool, passed through first feeder rolls, then heated in a heater (primary heater), cooled on a cooling rail, passed through a false twist spindle and second feeder rolls arranged thereafter, and so-called take-off feeder rolls before being finally wound onto a yarn spool. The false twist spindle is used to highly twist the multifilament yarn temporarily in one working process, i.e., to produce a twist of the multifilament yarn or the individual filament yarns by transferring an axial torque to the filament yarns. This temporary twist (torque state) is designated as false twist (FD). As a result of the twisting, a rotation back pressure is formed which extends back into the heater (twist zone) whereby it is possible for the torque state of the filament yarn to be thermally fixed by heating and cooling before the false twist spindle. After the false twist spindle, the twisting is then released again. As a result of the thermal fixing accomplished in the torque state, the yarn has the desired crimped structure.
Very high production speeds can be achieved by using a friction false twist spindle as the false twist spindle. In these false twist spindles the filament yarn is indirectly driven using friction surfaces. As a result of the smaller diameter of the thread compared to the spindle, i.e., to a disk of a disk friction unit, for example, a high transmission ratio is achieved between the revolution of the disk and the twisting of the filament yarn. A triaxial disk friction unit is especially suitable for this purpose. Thus, predominantly friction false twist spindles, especially triaxial disk friction units and also so-called nip twisters which transfer a torque to the filament yarns by means of crossed belts are used as false twist spindles. Such a disk friction unit is disclosed in DE 3743708 A1 for example. A nip twister is disclosed in JP 06184848. The imparting of twist by means of friction makes it possible to achieve very high rotation speeds and therefore also high production speeds. If the friction relationships between the filament yarns and the false twist spindle vary, that is if process fluctuations or instabilities occur, this then results in a non-uniform yarn structure or defects in the yarn and thus in loss of quality in the yarn produced. Such defects or disturbances can, for example, result from disturbances in the spinning mill, from non-uniform application or non-uniform adjustment of the spinning preparation on the thread surface, from temperature fluctuations during texturing or from contamination e.g. in the heater and/or in the cooling rail. The disturbances can bring about a so-called ballooning of the yarn which occurs particularly at high rotation speeds and the high thread tensions associated therewith. Ballooning of the yarn results in an uncontrolled run of thread and fluctuations in the thread tension. As a result, the thread can, for example, jump over the disk surface of the false twist spindle. This twist slippage leads to a twist deficit inside the twist zone, i.e., the twist density, that is the number of twists per unit length of filament yarn fluctuates. The thread to be processed can thus pass in sections through the false twist spindle without twisting. This results in short closed yarn sections, so-called “tight spots” and long non-uniformly textured yarn sections which is called surging. During surging the thread tension increases abruptly whereby the equilibrium of forces in the false twist spindle is destroyed. Zones are formed in the thread without twist. In addition, the stretching values fluctuate and the dyeing is unsatisfactory.
Texturing speeds of over 300 m/min can be achieved with friction false twist spindles. The lengths of the heating and cooling zones in the texturing zone are adapted to these texturing speeds in order to ensure sufficient thermal fixing of the crimping. If the total length of the texturing zone is 5–6 m, the phenomenon of surging occurs particularly frequently in conjunction with the friction false twist spindles which operate by force locking. In force-locking false twist spindles according to the prior art, the twist density produced cannot be controlled very exactly, and this results in process-technology production limitation of the surging and therefore ballooning of the thread in the twist zone with associated thread tension fluctuations which in turn results in twist fluctuations. The stability limit of the process is influenced on the one hand by the geometry of the texturing zone, e.g., its length, deflection points, thread support etc. and on the other hand by the quality of the feed material, e.g., its uniformity, preparation etc. that is by process fluctuations which occur.
Another factor which limits the production speed in false twist texturing processes is the unwinding speed of the yarns, e.g. a partially oriented multifilament yarn (POY), from the feeding spool. Higher unwinding speeds result in stronger variations of the yarn tension (thread tension) in the area of the run of thread after the feeding spool (unwinding region). This follows from the properties of the known “balloon” formed by the yarn during unwinding.
A false twist texturing process always with unwinding a POY yarn from a feeding spool. The yarn is taken off from the feeding spool by the rotating movement of a take-off roll (feeder rolls). The take-off roll usually comprises a main roll driven by a motor and a passive separating roll which defines the defines the geometry of windings of the yarn around the main roll. In order to avoid variations of thread tension in the unwinding region, it is known to exert a pressing force on the main roll by means of a nip roller, that is another roller. The yarn is thereby clamped between the main roller and the further roller, with the result that the yarn is supplied to the texturing process at the tangential speed of the main roller. Various pre-tensioning systems arranged between the feeding spool and the feeder rolls are further known to increase and stabilise the thread tension of the yarn during take-off from the feeding spool.
The variations of the thread tension, e.g. of the POY yarn taken off from the feeding spool, essentially have the following causes:                Since the diameter of the feeding spool decreases with time as a result of the unwinding of the thread, the geometry of the “balloon” formed by the yarn movements along the spool axis varies accordingly which influences the thread tension.        As a result of increasing the production speed and thus the unwinding speed of the yarn from the feeding spool, centrifugal forces acting on the yarn in the “balloon” increase, which increases the thread tension. These problems appear particularly severely especially with polyamide (PA) yarns.        
In the known devices for avoiding fluctuations of thread tension during texturing processes, the thread tension of the yarn can only be regulated before the start of the manufacturing process. This is not sufficient to avoid the variations of the thread tension during the manufacturing process which have been described.
The document CH 691 386 A discloses a device and a method for texturing a running thread. The tension of the thread entering into a texturing nozzle is detected and the twisting effect in the texturing nozzle is accordingly controlled.
It is further known from EP 0 875 479 A1 to measure and regulate the thread tension of individual yarns during yarn conditioning methods associated with winding processes, e.g., oiling, dyeing or stretching of yarns. In this case, the thread tension of a yarn is measured in its thread run after a conditioning device and the thread tension of the controlled yarn is kept constant before its spooling, that is at the end of the conditioning process, in a predetermined range of thread tension by means of a thread tension regulating module according to a control value derived from the measured thread tension. The thread tension regulating module comprises a brake and a controllable feeder roll by which means the thread tension is regulated by braking and/or accelerating the thread speed of the yarn.
It is the object of the invention to provide a yarn quality assurance method which avoids the disadvantages of the prior art and in particular, makes it possible to achieve a high process speed with minimal downtimes of the yarn processing machine.